单元测试mock当前时间

namespace Common.Helper
{
    /// <summary>
    /// 获取当前时间
    /// However, when unit testing with static state, always remember to tear down your fixture by calling .
    /// </summary>
    public abstract class TimeProvider
    {
        private static TimeProvider current =
            DefaultTimeProvider.Instance;
        public static TimeProvider Current
        {
            get { return TimeProvider.current; }
            set
            {
                if (value == null)
                {
                    throw new ArgumentNullException("value");
                }
                TimeProvider.current = value;
            }
        }

        public abstract DateTime Now { get; }

        public static void ResetToDefault()
        {
            TimeProvider.current = DefaultTimeProvider.Instance;
        }
    }
    public class DefaultTimeProvider : TimeProvider
    {
        private readonly static DefaultTimeProvider instance =
            new DefaultTimeProvider();

        private DefaultTimeProvider() { }

        public override DateTime Now
        {
            get { return DateTime.Now; }
        }

        public static DefaultTimeProvider Instance
        {
            get { return DefaultTimeProvider.instance; }
        }
    }
}

var now = TimeProvider.Current.Now;

var timeMock = new Mock<TimeProvider>();
timeMock.SetupGet(tp => tp.Now).Returns(new DateTime(2010, 3, 11));
TimeProvider.Current = timeMock.Object;